An Analog Tutorial: Current sources

I was at Burr-Brown in 1988 and I knew Mark Stitt well. He was a mentor to me and I was devastated at his untimely death. He had written an amazing Burr-Brown Current Source app note1 that I have referenced at the end of this tutorial. I will be bringing you a series of Analog and Power basics on Planet Analog with links to EDN with more in-depth articles on these different tutorial topics.

So first, what’s a current source?

A basic current source is a circuit that simply provides current to a load.

Here is a simple single Bipolar Junction Transistor (BJT) transistor current source in Figure 1:

Figure 1

Figure 1: A simple current source made from a single BJT2 (Image courtesy of Reference 2)

James Bryant, retired Analog Devices ‘guru’ says, “…current outputs offer advantages in a number of situations, including analog current-loop signaling (0 mA to 20 mA and 4 mA to 20 mA) in high-noise environments, and level shifting an analog signal across a large potential difference without the use of optical or magnetic isolation techniques.” (See Reference 4)

The Current Mirror)

Now let’s look at a two-transistor current source called a Current Mirror. Here the two transistors, Q1 and Q2 , are matched in Figure 2:

Figure 2

Here Q1 is connected as a diode with the short from collector to base. The two transistors are matched with equal VBE , IB , and IC . R1 > sets the Reference current IREF (Image courtesy of Reference 3)

A “Full Wilson” architecture

The REF200 has a “Full Wilson” architecture which gives high accuracy when designed as an IC with laser trimming of resistors in Figure 3.

Figure 3

The Wilson Current Source is another three-transistor architecture current source. All transistors are identical (this is easy in an IC on a monolithic substrate)(See Reference 3)

Adding another transistor (T4) will improve the accuracy and dynamic range of the current source as seen in Figure 4.

Figure 4

Adding T4 will improve the accuracy and the dynamic range of the Wilson Current Mirror (Image courtesy of Reference 4)

This current source uses lower value resistors in the low kOhm range which is good for IC designs since 1MOhm resistors occupy a pretty large area on an IC. See Figure 5.

Figure 5

The Widlar Current Source is commonly used in the front end of the differential transistor pair in many Op Amps. (Image courtesy of Reference 3)

A “Howland” Current Source4

The Howland Pump has a bipolar output, where the previous architectures above were unipolar. See Figure 6.

Figure 6

The Howland Pump: Beware that this architecture needs precisely matched resistors which is easy with laser trimming in an IC but not as easy in a discrete circuit design. (Image courtesy of Reference 4)

A low cost bipolar current source with discrete amplifiers and resistor4 .

We can also design a Bipolar-current circuit using an Op Amp, An Instrumentation Amp, and a resistor to sense the output current level in a feedback configuration. See Figure 7.